The present invention relates to a device for introducing gases into molten metal and relates also to their operation.
In many metallurgical operations gas is introduced into a charge of molten metal. Some of these operations are carried out at atmospheric pressure, e.g. degassing, mixing a melt with variable additions or reactive slags, or introducing a gas in view of changes in the metallographic structure of the treated metal after casting. Other operations are carried out at reduced pressure, for example vacuum-degassing by introducing a flushing gas. The total volume of gas used can vary in other areas, for example, between 60 and 600 liters per ton of treated aluminum or 30 to 300 liters per ton of steel.
A frequently used method is to introduce a gas into the melt via a lance dipping into the melt. This lance, which is usually provided with a protective coating of refractory material, is connected via a reduction valve to a conventional container of compressed gas. The lance, however, provides poor distribution of the gas in the melt. It also cools the melt somewhat and is subject to a relatively fast rate of wear so that the refractory material breaks off easily, giving rise to undesired inclusions in the metal.
The suggestion has been made to provide porous refractory parts on the base or walls of the container (holding crucible etc.) and to connect the inner face of these parts to a source of compressed gas which then enters the melt via the pores in them. Known are porous refractory elements which can be incorporated in the walls of the container and provide adequate sealing. It can happen however that damage occurs at some part of the porous element or the joint between the element and the rest of the container wall resulting in an improper seal. Although such cases occur relatively seldom in practice, when they do, they cause a disastrous amount of damage to the installation.
The object of the present invention is to find a device and a method to introduce gases into molten metal such that a maximum gas/melt interface is achieved, therefore ensuring as complete as possible reaction of the gas with the impurities in the melt, which is equivalent to achieving a maximum efficiency in the use of the gas introduced to the melt. In achieving this the disadvantages of the present state of the art should be avoided as much as is possible i.e. the poor gas distribution observed when using lances should be improved upon, contamination of the melt by material from the lance prevented and the cooling of the melt by the gas introduced should be reduced. Also, the difficulties which occur on introducing gas via porous bodies in the wall of the container should be overcome, in particular loss of gas and metal through cracks and gaps between the wall and the porous elements. Such a process should allow the melt to be treated with an optimum mixture of gases which are expensive or potentially hazardous; this allows the consumption of such gases to be reduced, at the same time maintaining the same quality of purified melt.